1. Field of the Invention
The present invention relates to a method of controlling power of a laser diode, and more particularly, to a method and apparatus for controlling power of a laser diode, which can obtain an output image with high quality by controlling driving voltage of the laser diode that scans a laser beam on a photosensitive drum so as to compensate for an optical power deviation between both sides of the photosensitive drum.
2. Description of the Related Art
In general, a laser printer is an apparatus that forms an image using a laser beam, emitted from a laser diode in response to a video signal of an input image, on a photosensitive drum and transfers a resultant latent image formed on the photosensitive drum onto a medium, such as a piece of paper, thereby realizing the input image.
FIG. 1 is a block diagram of a conventional apparatus for driving a laser diode. Referring to FIG. 1, the conventional apparatus includes an image processing unit 100, a pulse width modulation (PWM) unit 110, a digital-to-analog conversion unit 130, a laser diode driving unit 120, and a laser scanning unit 140 that comprises a laser diode and a light receptor, such as a photo diode.
The image processing unit 100 determines the size of each dot to be printed by applying a resolution algorithm to input binary data.
The PWM unit 110 generates a pulse signal based on information on the size of each dot to be printed.
The digital-to-analog conversion unit 120 receives the pulse signal from the PWM unit 110 and converts the received pulse signal, which is a digital signal, into an analog signal for driving the laser diode.
The laser diode driving unit 130 receives the analog signal from the digital-to-analog conversion unit 120 and drives the laser diode in the laser scanning unit 140 using the received analog signal.
FIGS. 2A-2D are waveform diagrams illustrating signals input to/output from the elements of the conventional apparatus for driving a laser printer of FIG. 1. Referring to FIGS. 2A-2D, binary data, which consists of “1”s and “0”s, is input to the image processing unit 100. The image processing unit determines 100 the size of each dot to be printed (where each dot to be printed may have a size of, for example, 0-255) by applying the resolution algorithm to the input binary data and outputs information on the size of each dot to be printed to the PWM unit 110.
The PWM unit 110 converts the information on the size of each dot to be printed into a video signal through PWM and outputs the video signal to the digital-to-analog conversion unit 120. The laser diode driving unit 130 adds a predetermined level of laser diode driving voltage to the video signal, thereby driving the laser diode.
However, even though the laser diode driving voltage applied to the laser diode is maintained at a constant level, optical power accumulated on the surface of the photosensitive drum may vary for many reasons. Therefore, it is necessary to maintain the optical power accumulated on the surface of the photosensitive drum at a constant level.
One of the reasons for the variation of the optical power accumulated on the surface of the photosensitive drum is the structural mechanism of the laser scanning unit 140. In other words, even when the laser diode scans a laser beam with a constant level of power, the optical power accumulated on the surface of the photosensitive drum varies from positions of the photosensitive drum.
FIG. 3 is a diagram illustrating optical power accumulated on the photosensitive drum that differs from position of the surface of the photosensitive drum. Referring to FIG. 3, optical power is lower at either side of the photosensitive drum than at the center of the photosensitive drum due to a polygonal mirror and a lens installed in the laser scanning unit 140. Therefore, the quality of printing at either side of printing paper differs from the quality of printing at the center of the printing paper.